An electrical measurement system generally comprises a measurement instrument, such as an oscilloscope or logic analyzer, and a probe assembly. The probe assembly comprises a probe body, a probe tip that extends from the probe body to contact a test point of a device under test, and a cable for connecting the probe body to the measurement instrument. The probe body may incorporate circuitry for interfacing the probe tip to the cable, for example impedance-matching circuitry, or other types of circuits, such as an attenuator. An active probe, such as the Tektronix P6202, incorporates active circuit elements in the probe body, and these require that the probe circuitry be connected to a power supply.
In order to enable a measurement instrument to provide accurate measurement results, it is necessary that the signal acquisition components of the system be calibrated. The signal acquisition components of an electrical measurement system include the probe assembly and the input circuits of the measurement instrument. In the case of a digital storage oscilloscope, these circuits include an analog-to-digital converter (ADC) for converting the analog electrical signal received from the probe assembly into digital form for storage in an acquisition memory. At present, calibration is accomplished by connecting the probe tip to a signal having a predetermined waveform and adjusting calibration circuits of the measurement instrument so that the measurement result actually provided by the measurement instrument is the same as the expected measurement result. Good measurement practice dictates that a measurement instrument be calibrated at short enough intervals to take account of changes in signal acquisition conditions, such as the temperature of the probe assembly. However, calibration is generally regarded as an inconvenient chore and therefore many users do not calibrate their instruments sufficiently frequently.